Switched mode power supply responsive to current derived from voltage across energy transfer element input

ABSTRACT

A switched mode power supply having a regulated reflected voltage. In one embodiment, a switched mode power supply includes a power supply regulator coupled between a positive input supply rail of the power supply and a primary winding of an energy transfer element. The reflected voltage across the primary winding of the transfer element is related to the output voltage across a secondary winding of the energy transfer element according to the turns ratio of the energy transfer element. The power supply regulator is coupled to regulate the reflected voltage across the primary winding, thereby regulating the output voltage across secondary winding. In one embodiment, the reflected voltage across the primary winding is sensed through a current representative of the reflected voltage received by the power supply regulator.

RELATED APPLICATION

This application is a continuation-in-part application of applicationSer. No. 09/517,461, filed Mar. 2, 2000 U.S. Pat. No. 6,233,161,entitled “Switched Mode Power Supply Responsive To Voltage Across EnergyTransfer Element,” and assigned to the Assignee of the presentapplication.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to power supplies and, morespecifically, the present invention relates to a switched mode powersupply.

2. Background Information

Electronic devices use power to operate. Linear power supplies oradapters are widely used to power electronic products as well as chargebatteries used to power mobile products such as for example wirelessphones, palm top computers, toys, etc. due to their low cost. However,linear adapters typically include 50-60 Hz transformers, which result inlinear power supplies that are very bulky and in efficient.

Switched mode power supplies are commonly used due to their highefficiency and good output regulation to supply power to many of today'selectronic devices. Switched mode power supplies offer the benefits ofsmaller size, weight, high efficiency and low power consumption at noload in many applications relative to linear power supplies. However,known switched mode power supplies are generally more expensive thantheir linear power supply counterparts at low power levels, for examplebelow 5 watts, due to the relatively high number and cost of componentsand the complexity of circuitry. Consequently, linear power supplies arestill commonly used in applications having power levels below 5 watts,even though the linear power supplies are bulky and inefficient.

SUMMARY OF THE INVENTION

Switched mode power supplies are disclosed. In one embodiment, a powersupply includes an energy transfer element having an energy transferelement input and an energy transfer element output, a power switchcoupled between the energy transfer element input and a positive inputsupply rail, and control circuitry coupled to the energy transferelement input and to the power switch. The control circuitry is coupledto switch the power switch in response to a current derived from thevoltage at the energy transfer element input. Additional features andbenefits of the present invention will become apparent from the detaileddescription, figures and claims set forth below.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention detailed illustrated by way of example and notlimitation in the accompanying figures.

FIG. 1 is a schematic illustrating one embodiment of a power supplyincluding a power supply regulator coupled to regulate a reflectedvoltage from a primary winding of an energy transfer element inaccordance with the teachings of the present invention.

FIG. 2 is a schematic illustrating another embodiment of a power supplyincluding a power supply regulator coupled to regulate a reflectedvoltage from a primary winding of an energy transfer element inaccordance with the teachings of the present invention.

FIG. 3 is a diagram illustrating a power supply regulator includingcircuitry to regulate a reflected voltage from an energy transferelement of a power supply in accordance with the teachings of thepresent invention.

FIG. 4 is a diagram illustration the output current/voltage relationshipfor several embodiments of power supply regulators in accordance withthe teachings of the present invention.

FIG. 5 is a schematic illustrating yet another embodiment of a powersupply including a power supply regulator coupled to regulate a voltageat the output of the energy transfer element using a current derivedfrom the voltage across the energy transfer element input in accordancewith the teachings of the present invention.

FIG. 6 is a diagram illustration the output current/voltage relationshipfor other embodiments of power supply regulators in accordance with theteachings of the present invention.

FIG. 7 is a schematic illustrating still another embodiment of a powersupply including a power supply regulator coupled to regulate a voltageat the output of the energy transfer element using a current derivedfrom the voltage across the energy transfer element input in accordancewith the teachings of the present invention.

FIG. 8 is a diagram illustration the output current/voltage relationshipfor still other embodiments of power supply regulators in accordancewith the teachings of the present invention.

DETAILED DESCRIPTION

A novel switched mode power supply regulator is disclosed. In thefollowing description, numerous specific details are set forth in orderto provide a thorough understanding of the present invention. It will beapparent, however, to one having ordinary skill in the art that thespecific detail need not be employed to practice the present invention.In other instances, well-known materials or methods have not beendescribed in detail in order to avoid obscuring the present invention.

In one embodiment, the present invention provides a simple low costswitched mode power supply. The present invention reduces the cost andcomponent count of a switched mode power supply, which enables thepresently described power supply to be cost effective when used in lowpower applications, including applications with power levels below 5watts. Therefore, various embodiments of the power supply of the presentinvention may be used as cost effective alternatives to replace AC to DClinear adapters and battery chargers that use 50-60 Hz transformers,such as for example those power supplies used for wireless phonechargers or the like.

In one embodiment, the presently described power supply reduces thecomponent count of known switched mode power supplies by regulating thereflected voltage from the energy transfer element. The energy transferelement may be a transformer, an inductor, coupled inductors, or thelike. For instance, using a transformer having a primary winding and asecondary winding, one embodiment of the present invention includes apower supply regulator coupled between the positive input supply rail ofthe power supply and the primary winding. The power supply regulator ofthe present invention is therefore able to regulate the reflectedvoltage from the primary winding, which is related to the output voltageon the secondary winding through the turns ratio of the transformer.Thus, the switched mode power supply regulates the reflected voltagefrom the primary winding, which regulates the output voltage on thesecondary winding. The presently described power supply in oneembodiment provides regulated output without feedback circuitry coupledto the secondary winding to monitor the output voltage directly. Thisenables the present invention to have a lower component count than knownswitched mode power supplies.

To illustrate, FIG. 1 is a schematic showing one embodiment of a powersupply 101 in accordance with the teachings of the present invention. Asshown, one embodiment of power supply 101 includes a flyback converterhaving alternating current (AC) input 103 and a direct current (DC)output 155. A rectifier 107 is coupled to AC input 103 through aresistor 105. In one embodiment resistor 105 is a fusible resistor thatis used for fault protection in place of a fuse for lower cost. Inanother embodiment, a fuse or the like is used in place of resistor 105.Rectifier 107 converts the AC from AC input 103 to DC, which is thenfiltered in one embodiment by capacitors 113 and 115, which are coupledin parallel across rectifier 107. In one embodiment, an inductor 109 iscoupled between capacitors 113 and 115 such that a π filter is formed tofilter electromagnetic interference (EMI) generated by power supply 101.A resistor 111 is coupled in parallel with inductor 109 betweencapacitors 113 and 115 in one embodiment to damp inductor resonance frominductor 109 that can cause peaks in the EMI spectrum.

In one embodiment, a low frequency(e.g. 50 Hz or 60 Hz mains frequency),high voltage AC is received at AC input 103 and is converted to highvoltage DC with rectifier 107 and capacitors 113 and 115. Thus, apositive input supply rail 117 and negative input supply rail 119 areprovided at opposite ends of capacitor 115. The high voltage DC is thenconverted to high frequency (e.g. 20 to 300 kHz) AC, using a switchedmode power supply regulator 121. This high Frequency, high voltage AC isapplied to an energy transfer element 145, such as for example atransformer, to transform the voltage, usually to a lower voltage, andto usually provide safety isolation. The output of the energy transferelement 145 is rectified to provide a regulated DC output at DC output155, which may be used to power an electronic device. In the embodimentdepicted, energy transfer element 145 is a transformer or coupledinductor having an input primary winding 161 magnetically coupled to anoutput secondary winding 163.

In one embodiment, the power supply regulator 121 utilized in a powersupply in accordance with the teachings of the present inventioncomprises a single monolithic chip, which may be for example a knownTINYSwitch power supply regulator of Power Integrations, Incorporated,of San Jose, Calif. In another embodiment, a power supply may utilizefor example a known TOPSwitch power supply regulator of PowerIntegrations, Incorporated, of San Jose, Calif., in accordance with theteachings of the present invention. In one embodiment, power supplyregulator 121 includes an electrical terminal 123 coupled to thepositive input supply rail 117 and an electrical terminal 129 coupled toprimary winding 161 of energy transfer element 145. Thus, power supplyregulator 121 is coupled in series with primary winding 161.

In one embodiment, power supply regulator 121 includes a power switchcoupled between electrical terminals 123 and 129 and associated controlcircuitry coupled to control or switch the power switch. In oneembodiment, the associated control circuitry includes an oscillator, alatch, current limit circuitry, control logic, start-up and protectioncircuitry. In one embodiment, the power switch within power supplyregulator 121 is turned on every cycle by the oscillator by setting thelatch, and is turned off when either the current through the powerswitch reaches a current limit value or if a maximum on time is reached,by resetting the latch.

In one embodiment, the power supply regulator 121 also includes a biassupply electrical terminal 127. In one embodiment, a capacitor 131 iscoupled between terminals 127 and 129 to provide energy storage and highfrequency bypassing.

In operation, energy is transferred to secondary winding 163 from theprimary winding 161 in a manner controlled by the power supply regulator121 to provide the clean and steady source of power at the DC output155. When the power switch within power supply regulator 121 is on,input supply rail 117 is coupled to primary winding 161 and currentramps up in primary winding 161. When the power switch within powersupply regulator 121 is turned off, the current flow through primarywinding 161 is interrupted, which forces the voltages V1 157 on primarywinding 161 and V2 159 on secondary winding 163 to reverse. The reversalof voltages in V1 157 and V2 159 when the power switch is off allowsdiode 149 to conduct to deliver stored energy in the energy transferelement to DC output 155.

During the period that the power switch in power supply regulator 121 isoff and diode 149 conducts, the voltage V1 157 on primary winding 161 isa reflected voltage of voltage V2 159 on the secondary winding 163. Inone embodiment, the reflected voltage V1 157 is opposite in polarity tothe voltage V1 157 applied to primary winding 161 when the power switchin power supply regulator 121 is on and related to V2 159 by the turnsratio of the transformer of energy transfer element 145. To illustrate,assume for example that the transformer of energy transfer element 145has a 20:1 turns ratio. In this instance, if there is 5 volts (V2 159)across the secondary winding 163, the reflected voltage (V1 157) acrossthe primary winding 161 would be 100 volts during the period that thepower switch in power supply converter 121 is off and diode 149conducts. At this time, the polarity of reflected voltage V1 157 isreversed and the voltage at electrical terminal 129 is low relative tothe voltage at input supply rail 119.

In one embodiment, the reflected voltage V1 157 is used as feedback toprovide information to power supply regulator 121 through diode 143,resistor 141, capacitor 137, zener diode 135, resistor 136 andtransistor 133. When the power switch of power supply regulator 121 isturned off, the reversed polarity of the reflected voltage V1 157 acrossprimary winding results in diode 143 conducting. When the power switchof power supply regulator 121 is turned on, diode 143 no longerconducts.

In one embodiment, when the voltage is reflected in voltage V1 157,there is also a voltage spike in the reflected voltage when the powerswitch in power supply regulator 121 is switched off due to leakageinductance in primary winding 161, which is a part of the inductance ofthe primary winding 161 that is not coupled to secondary winding 163.The energy contained in this voltage spike is commonly referred to asleakage energy. The leakage energy, which is not coupled to secondarywinding 163, is clamped by zener diode 139. In another embodiment, aresistor-capacitor-diode (RCD) clamped circuit can be used instead ofzener diode 139. In that embodiment, a parallel combination of aresistor and capacitor is used in place of zener diode 139.

In one embodiment, a low pass RC filter is provided by resistor 141 andcapacitor 137 to filter the voltage spike caused by the leakage energy,which would otherwise represent an error in the reflected output voltagefeedback. After the voltage spike, the remaining voltage of reflectedvoltage V1 157 indicates the output voltage V2 159 divided by the turnsratio of the transformer of energy transfer element 145, ignoringforward voltage drops of diodes 143 and 149. In one embodiment, when theoutput of the RC filter of resistor 141 and capacitor 137, which is thevoltage across capacitor 137, exceeds the zenr voltage of zener diode135 plus the base to emitter voltage V_(BE) of transistor 133,transistor 133 is switched on. When transistor 133 is switched on, powersupply regulator 121 is disabled from switching by electrical terminal125 being pulled to a low voltage through transistor 133.

In this embodiment, the power supply regulator 121 is disabled fromswitching by the transistor 133 for a number of switching cycles, whichis a function of the output load. For example, with low load levels atthe DC output 155, the voltage across the capacitor 137 is slightlyhigher, which overdrives the transistor 133, keeping it on for a longerperiod of time. In this case, the power supply regulator 121 is disabledfor many switching cycles and only few in many switching cycles isenabled. Whereas, with high load levels at the DC output 155, thevoltage across the capacitor 137 is slightly lower, which provides lessoverdrive for the transistor 133, keeping transistor 133 on for ashorter period of time. Therefore, the power supply regulator 121 isdisabled for fewer switching cycles and enabled for a large number ofswitching cycles. This way the power delivered to the DC output 155 isregulated to maintain the voltage across capacitor 137 in a relativelynarrow range above the threshold set by the zener diode 135 and the baseto emitter voltage V_(BE) of transistor 133, independent of output loadconditions. In another embodiment, a known PWM regulator such as forexample a TOPSwitch power supply regulator (not shown) could be usedinstead of the power supply regulator 121. In this case, the voltageacross the capacitor 137 can be used to reduce the duty cycle of thepower switch within the TOPSwitch power supply regulator, when thevoltage reaches a threshold value. The duty cycle can be reduced from alarger value at just below this threshold to a lower value at a voltageslightly above the threshold such that the voltage across the capacitor137 is maintained within a narrow range above the threshold independentof the output load conditions.

Therefore, the output voltage V2 159 at DC output 155 of power supply101 is regulated in accordance with the teachings of present inventionby monitoring or regulating the reflected voltage V1 157 across primarywinding 161. By regulating the reflected voltage V1 157, the outputvoltage V2 159 is regulated. It is appreciated that output voltage V2159 is regulated by power supply 101 without the use of feedbackcircuitry coupled to DC output 155. Indeed, known flyback powerconverters often utilize circuitry such as opto-couplers or a separatefeedback winding to provide feedback information. Thus, the componentcount of the presently described power supply 101 is reduced compared toknown switched mode power supplies.

In one embodiment, the turns ratio of the transformer of energy transferelement 145 is designed to accommodate an output short circuit currentcondition in accordance with an internal current limit of the powerswitch of the power supply regulator 121. In one embodiment, a constantoutput current/constant output voltage characteristic is provided at DCoutput 155 by power supply 101 for applications such as for examplebattery charging. In one embodiment, power supply 101 includes aresistor 153 coupled across DC output 155 to provide a minimum load toimprove load regulation at no load.

FIG. 2 is a schematic of another embodiment of a power supply 201 inaccordance with the teachings of the present invention. As shown, oneembodiment of power supply 201 is a flyback converter having AC input203 and a DC output 255. Rectifier 207 is coupled to AC input 203through a resistor 205. Rectifier 207 converts the AC from AC input 203to DC, which is then filtered in one embodiment by capacitors 213 and215, which are coupled in parallel across rectifier 207. In oneembodiment, inductor 209 and resistor 211 are coupled in parallelbetween capacitors 213 and 215.

In one embodiment, low frequency, high voltage AC is received at ACinput 203 and is converted to high voltage DC with rectifier 207 andcapacitors 213 and 215 providing positive input supply rail 217 andnegative input supply rail 219 at opposite ends of capacitor 215. Thehigh voltage DC is then converted to high frequency AC using a switchedmode power supply regulator 221. This high frequency, high voltage AC isapplied to an energy transfer element 245 to transform the voltage,usually to a lower voltage, and to usually provide safety isolation. Theoutput of the energy transfer element 245 is rectified to provide aregulated DC output at DC output 255. In the embodiment depicted, energytransfer element 245 is a transformer or coupled inductors having aninput primary winding 261 magnetically coupled to an output secondarywinding 263.

In one embodiment, power supply regulator 221 includes a power switchcoupled between electrical terminals 223 and 229 and associated controlcircuitry coupled to control the power switch. In operation, energy istransferred to secondary winding 263 from the primary winding 261 in amanner controlled by the power supply regulator 221. When the powerswitch within power supply regulator 221 is on, input supply rail 217 iscoupled to primary winding 261 and current ramps up in primary winding261. When the power switch within power supply regulator 221 is turnedoff, the current flow through primary winding 261 is interrupted, whichforces the voltages V1 257 on primary winding 261 and V2 259 onsecondary winding 263 to reverse. The reversal of voltages in V1 257 andV2 259 when the power switch is off allows diode 249 to conduct todeliver stored energy in the energy transfer element to DC output 255.In addition, the reversal of voltages in V1 257 and V2 259 allows diode243 to conduct, which enables capacitor 237 to sample and hold thereflected voltage V1 257 across primary winding 261.

In one embodiment, electrical terminal 225 is a low impedance currentsense terminal that senses current through resistor 239. In theembodiment depicted, the current that flows through resistor 239 isresponsive to the voltage across capacitor 237, which is responsive toreflected voltage V1 257 across the primary winding 261 of energytransfer element 245. In the embodiment depicted, resistor 239,capacitor 237 and diode 243 form an RCD clamp, which clamps theinductive leakage voltage spikes that occur across primary winding 261when the power switch in power supply regulator 221 is switched off. Inone embodiment, a relatively slow diode is designed for diode 243 suchthat there is a relatively constant DC voltage across capacitor 237,which is substantially the same as the reflected voltage V1 257 acrossprimary winding 261. In one embodiment, the reflected voltage V1 257 isrelated by the turns ratio of energy transfer element 245 to the outputvoltage V2 259 across secondary winding 263. FIG. 3 is a block diagramillustrating one embodiment of a power supply regulator 321 inaccordance with the teachings of the present invention. In oneembodiment, power supply regulator 321 is one embodiment of a regulatorthat may be used in place of power supply regulator 221 of FIG. 2. Inone embodiment, power supply regulator 321 is included on a singlemonolithic chip having as few as three electrical terminals. Asillustrated in FIG. 3, power supply regulator 321 includes a powerswitch 365 coupled between electrical terminals 323 and 329. In oneembodiment, power switch 365 comprises a metal oxide semiconductor fieldeffect transistor (MOSFET). In one embodiment, power switch 365comprises an n-channel MOSFET having a drain coupled to terminal 323 anda source coupled to terminal 329. In one embodiment, terminal 323 isconfigured to be coupled to a positive input supply rail and terminal329 is configured to be coupled to an energy transfer element of a powersupply.

As shown in the embodiment depicted, power supply regulator 321 alsoincludes a current sensor 369 coupled to receive a current throughcurrent sense terminal 325. In one embodiment, the current receivedthrough the current sense terminal 325 is responsive to a reflectedvoltage from a energy transfer element of a power supply that powersupply regulator 321 is coupled to regulate. In one embodiment, powerswitch 365 is switched in response to the current received through thecurrent sense terminal 325. In addition, current sensor 369 provides inone embodiment a low impedance connection between current sense terminal325 and terminal 329. A control circuit 367 is coupled to current sensor369 and power switch 365 in one embodiment. As such, control circuit 367is coupled to control the switching of power switch 365 responsive tothe current coupled to be received through current sense terminal 325.

In one embodiment, power supply regulator 321 also includes a start-upcircuit 371 coupled to current sense terminal 325, terminal 323 andcontrol circuit 367. One embodiment of control circuit 367 includes avoltage mode or a current mode pulse width modulator (PWM) regulator orthe like to control the switching of power switch 365. In anotherembodiment, control circuit 367 includes an on/off control circuit, or avariable frequency circuit, or a cycle skipping circuit, or the like tocontrol the switching of power switch 365.

In one embodiment, a current limit circuit 368 is also included in powersupply regulator 321. As illustrated, current limit circuit 368 iscoupled to the drain and source of power switch 365 and coupled tocontrol circuit 367. In one embodiment, current limit circuit 368monitors the current that flows through power switch 365 when turned onby monitoring the drain to source voltage of power switch 365. In oneembodiment, the on resistance of power switch 365 is used as a currentsense resistor. In one embodiment, when the current that flows throughpower switch 365 reaches a current limit, control circuit 367 adjuststhe switching of power switch 365 accordingly such that that the currentthat flows through power switch 365 does not exceed the current limit.

In one embodiment, the current limit of the power switch 365 determinedby current limit circuit 368 is adjusted in response to the currentrepresentative of the reflected voltage received through current senseterminal 325. For example, in one embodiment, the current limit isadjusted from a lower value during start up of the power supply to ahigher value at a regulation threshold.

In one embodiment, a bias current used to power the circuitry of powersupply regulator 321 after start-up is also coupled to be receivedthrough current sense terminal 325. In one embodiment, a capacitor isconfigured to be coupled between current sense terminal 325 and terminal329. Referring briefly back to the embodiment illustrated in FIG. 2,this capacitor may correspond to capacitor 231 coupled between terminals225 and 229. In one embodiment, capacitor 231 also provides control loopcompensation for power supply 201. In another embodiment, the biascurrent used to power the circuitry of power supply regulator 321 may bederived from terminal 323. In this embodiment, a capacitor may becoupled between a separate bias supply electrical terminal (not shown)and terminal 329 for energy storage and high frequency bypassing.

Operation of an embodiment of power supply 201 utilizing a power supplyregulator 321 for power supply regulator 221 is as follows. Assume forthis illustration that terminals 223, 225 and 229 of power supplyregulator 221 correspond to terminals 323, 325 and 329, respectively, ofpower supply regulator 321. Referring to both FIGS. 2 and 3, at power-upor a beginning of a start-up period of power supply 201, start-upcircuit 371 in one embodiment is coupled to provide a current betweenterminal 323 and current sense terminal 325 to charge capacitor 231 toan adequate voltage to provide the bias current used to supply power topower supply regulator 321 for the duration of the start-up condition.In one embodiment, a current source (now shown) included within start-upcircuit 371 is activated to draw current from terminal 323 to chargecapacitor 231 through current sense terminal 325. After capacitor 231 issufficiently charged, the current source in start-up circuit 371 isdeactivated. When the sufficient voltage is reached in capacitor 231,the energy stored in capacitor 231 is used in one embodiment to operatepower supply regulator 321 long enough to complete the start-up of powersupply 201.

In another embodiment, an additional terminal (not shown) may beincluded for connection to a start-up energy storage capacitor, such asfor example capacitor 231. Alternatively, in this embodiment, the biascurrent used to power the power supply regulator 321 may be derived fromterminal 323 both during start-up and during normal operation afterstart-up. In either case, the capacitor coupled to the additionalterminal can also perform the function of high frequency bypassing.

During start-up of power supply 201, the current received throughcurrent sense terminal 325 representative of the reflected voltage V1257 from primary winding 261 is substantially zero. At this time, oneembodiment of current limit circuit 368 and control circuit 367 arecoupled to switch power switch 365 such that a limited amount of poweris delivered to secondary winding 263 to charge output capacitor 251,resulting in reflected voltage V1 257 eventually being large enough tocharge capacitor 237 to drive current through resistor 239 into currentsense terminal 325.

In one embodiment, after start-up, the current driven through resistor239 is also used to supply the bias current used to supply power topower supply regulator 321. In one embodiment, the current driventhrough resistor 239 to supply the bias current also includes currentresulting in the inductive leakage voltage spikes that occur acrossprimary winding 261 when power switch 365 is switched off. It isappreciated that known switched mode power supplies often simplydissipate the energy caused by leakage inductance. Thus, power supply201 has increased efficiency over known switched mode power suppliesbecause a part of the energy from the leakage inductance is utilized tosupply power to power supply regulator 321. In addition, a separate biaswinding on the energy transfer element 245 is not needed to provide thebias supply current, as is sometimes the case in known switched modepower supplies. Thus, power supply 201 operates with fewer componentsthan known switched mode power supplies, which reduces cost.

In one embodiment, as the current representative of the reflectedvoltage V1 257 driven through resistor 239 into current sense terminal325 increases, power supply regulator 321 is coupled to increase thepower level delivered to DC output 255 such that a substantiallyconstant output current is delivered by DC output 255, which issubstantially independent of the output voltage across DC output 255. Inone embodiment, the power level delivered to the DC output 255 ischanged by changing the current limit determined by current limitcircuit 368 of power switch 365 from a lower value at start-up as afunction of the current through resistor 239 to a higher value at theregulation threshold.

In one embodiment, when the current representative of the reflectedvoltage V1 257 driven through resistor 239 reaches the regulationthreshold, power supply regulator 321 is coupled to reduce the powerdelivered by power switch 365 such that reflected voltage V1 257 ismaintained very close to this level, which drives current approximatelyequal to the regulation threshold through resistor 239. Accordingly, theoutput voltage V2 259 is maintained at a voltage related to reflectedvoltage V1 257 based on the turns ratio of energy transfer element 245,the regulation threshold value and the value of resistor 239.

FIG. 4 is a diagram 401 illustrating the relationships of output currentand output voltage of several embodiments of a power supply inaccordance with the teachings of the present invention. As illustratedin characteristic curve 403 of FIG. 4, one embodiment of a power supplyin accordance with the teachings of the present invention exhibits asubstantially constant output current/constant output voltagecharacteristics. That is, as output current increases, output voltageremains substantially constant until the output current reaches anoutput current threshold. As the output current approaches the outputcurrent threshold, output voltage decreases as the output currentremains substantially constant over the drop in output voltage. It isappreciated that the constant output voltage/constant output currentcharacteristics of one embodiment of the present invention are suitablefor battery charger applications or the like.

In another embodiment, characteristic curve 405 shows that oneembodiment of a power supply in accordance with the teachings of thepresent invention has a substantially constant voltage until the outputcurrent reaches an output current threshold. After the output currentthreshold is reached, the output current increases as output voltagedecreases. In yet another embodiment, characteristic curve 407 showsthat another embodiment of a power supply in accordance with theteachings of the present invention has a substantially constant voltageuntil the output current reaches an output current threshold. After theoutput current threshold is reached, the output current decreases asoutput voltage decreases.

In one embodiment, control circuit 367 in FIG. 3 provides constantoutput voltage control by reducing the duty cycle of power switch 365when current sensor 369 senses that the current received at currentsense terminal 325 has reached the regulation threshold. In oneembodiment, substantially accurate regulation is provided by powersupply regulator 321 by control circuit 367 causing relatively largeduty cycle changes in power switch 365 for relatively slight changes incurrent sensed by current sensor 369 above the regulation threshold. Asa result, the current received through current sense terminal 325remains substantially constant near the regulation threshold in oneembodiment of the present invention.

In one embodiment, the constant output voltage value of characteristiccurve 403 in FIG. 4 is determined by the value of resistor 239 and theturns ratio of the transformer of energy transfer element 245 in FIG. 2for a given regulation threshold current value. In one embodiment, theconstant output current value of characteristic curve 403 in FIG. 4, isdetermined by the current limit of power switch 365 at the regulationthreshold, the turns ratio of the transformer of energy transfer element245, and the inductance of primary winding 261. It is appreciated thatit is possible to select any combination of output voltage and constantcurrent value within the power range of power supply regulator 321 byselecting an appropriate primary inductance and turns ratio for thetransformer of energy transfer element 245 and the value of resistor239.

Thus, in one embodiment, constant output voltage/constant output currentcharacteristics are provided by power supply 201 through regulation ofthe reflected voltage V1 257.

FIG. 5 is a schematic of another embodiment of a power supply 501 inaccordance with the teachings of the present invention. As shown, oneembodiment of power supply 501 is a flyback converter having AC input503 and a DC output 555. Rectifier 507 converts the AC from AC input 503to DC, which is then filtered in one embodiment by capacitors 513 and515, which are coupled in parallel across rectifier 507. In oneembodiment, the fundamental operation and transfer of energy realizedwith power supply 501 of FIG. 5 is similar to that described above inconnection with power supply 201 of FIG. 2.

In one embodiment, the reversal of voltages in V1 557 and V2 559 when apower switch included in power supply regulator 521 is off allows diode549 to conduct to deliver stored energy in the energy transfer element545 to DC output 555. In addition, the reversal of voltages in V1 557and V2 559 allows diode 543 to conduct, which enables capacitor 537 tosample and hold the reflected voltage V1 557 across primary winding 561.

As shown in the embodiment depicted in FIG. 5, power supply 501 includesan opto-coupler including an output transistor 541 and an input lightemitting diode (LED) 556. Output transistor is coupled to an electricalterminal 525 of power supply regulator 521 and is coupled to primarywinding 561 through diode 543 and resistor 540. Input LED 556 is coupledto DC output 555 and coupled to secondary winding 563 through diode 549.In one embodiment, electrical terminal 525 of power supply regulator 521is a low impedance current sense terminal that senses current throughopto-coupler output transistor 541. The current that flows throughresistor 540 and opto-coupler output transistor 541 is derived from thevoltage V1 557 across the input of energy transfer element 545 orprimary winding 561.

In one embodiment, the current that flows through the opto-coupleroutput transistor 541 is responsive to the voltage difference betweenthe DC output voltage 555 and the sum of the opto coupler input LED 556forward voltage drop and the zener voltage of a zener diode 554 coupledbetween input LED 556 and DC output 555 as shown in FIG. 5. It isappreciated that in another embodiment, the zener 554 could be replacedwith another component providing a reference voltage to determine theoutput regulation voltage. In one embodiment, zener diode 554 is biasedwith current flowing through a resistor 553 coupled across input LED 556as shown in FIG. 5 to improve the dynamic impedance of zener diode 554.In one embodiment, the combination of resistors 540 and 539 coupledacross capacitor 537 form a voltage divider to insure that voltage V1557 across the primary winding 561 at the input of energy transferelement 545 does not exceed the voltage rating of the output transistor541.

In one embodiment, the power supply 501 of FIG. 5 provides asubstantially constant output voltage of a value dependent at least inpart on the design choice of zener diode 554 at output power levelsbelow the peak output power capability of the power supply 501. FIG. 6illustrates two examples of substantially constant output voltagecharacteristics, 603 and 604 exhibited by various embodiments of powersupplies in accordance with the teachings of the present invention. Thepeak output power capability of the power supply is shown ascharacteristic curve 610 in FIG. 6, the value of which may be dependenton a number of power supply variables including the specification of thepower supply regulator 521, value of the AC input voltage 503, thecapacitance values of capacitors 513 and 515 in addition to the designof energy transfer element 545. It is appreciated that the peak outputpower capability in general of a flyback power supplies and theirdependence on the above variables is well documented in literature inthe art.

As shown in FIG. 6, the intersection of the substantially constantoutput voltage characteristics, 603 or 604 with the power supply peakoutput power characteristic, 610, defines the point where a power supplyaccording to the teachings of the present invention is no longer capableof supplying more output power. At this point, the power supply outputvoltage falls out of regulation and the feedback current through outputtransistor 541 falls substantially to zero. In one embodiment of thepower supply regulator 521, at the point where the peak output power isreached, the output voltage is allowed to fall to substantially zero asshown by characteristics 608 and 609.

In one embodiment, the power supply regulator 521 will periodicallyrestart to establish the output power requirement at the DC output 555of the power supply 501. If the output power requirement continues toexceed the maximum output capability of the power supply 501, the DCoutput 555 voltage is again allowed to fall to substantially zero asshown by characteristics 608 and 609. In one embodiment, this process isrepeated periodically.

In one embodiment, the power supply regulator 521 is designed tomaintain continuous operation when DC output 555 voltage regulation islost. In this embodiment, the output characteristic is defined by thepeak output power characteristic curve of 610.

FIG. 7 is a schematic of another embodiment of a power supply 701 inaccordance with the teachings of the present invention. As shown, oneembodiment of power supply 701 is a flyback converter having AC input703 and a DC output 755. Rectifier 707 converts the AC from AC input 703to DC, which is then filtered in one embodiment by capacitors 713 and715, which are coupled in parallel across rectifier 707. In oneembodiment, the fundamental operation and transfer of energy realizedwith power supply 701 of FIG. 7 is similar to that described above inconnection with power supply 201 of FIG. 2.

In one embodiment, the reversal of voltages in V1 757 and V2 759 when apower switch included in power supply regulator 721 is off allows diode749 to conduct to deliver stored energy in the energy transfer element745 to DC output 755. In addition, the reversal of voltages in V1 757and V2 759 allows diode 743 to conduct, which enables capacitor 737 tosample and hold the reflected voltage V1 757 across primary winding 761.

As shown in the embodiment depicted in FIG. 7, power supply 701 includesan opto-coupler including an output transistor 741 and an input lightemitting diode (LED) 756. Output transistor is coupled to an electricalterminal 725 of power supply regulator 721 across resistor 739 and iscoupled to primary winding 761 through diode 743 and resistor 740. InputLED 756 is coupled to DC output 755 and coupled to secondary winding 763through diode 749. In one embodiment, electrical terminal 725 of powersupply regulator 721 is a low impedance current sense terminal thatsenses current through resistor 740, and the parallel combination ofresistor 739 and opto-coupler output transistor 741. The current thatflows into the current sense terminal 725 is derived from the voltage V1757 across the primary winding 761 or the input of energy transferelement 745.

In one embodiment, the current that flows through the opto-coupleroutput transistor 741 is responsive to the voltage difference betweenthe DC output voltage 755 the sum of the opto coupler input LED 756forward voltage drop and the zener voltage of zener diode 754 coupledbetween input LED 756 and DC output 755 as shown in FIG. 7. In oneembodiment, zener 754 is biased with current flowing through a resistor753 coupled across input LED 756 to improve the dynamic impedance of thezener diode 754. It is appreciated that in another embodiment, the zener754 could be replaced with another component providing a referencevoltage to determine the output regulation voltage. In one embodiment,the combination of resistors 740 and 739 form a voltage divider toinsure that voltage V1 757 across the primary winding 761 at the inputof energy transfer element 745 does not exceed the voltage rating of theoutput transistor 741.

In one embodiment, when the output voltage 755 is below the sensevoltage (out of voltage regulation), the opto-coupler output transistor741 is off and the current that flows through resistor 739 and resistor740 is responsive to the voltage across capacitor 737, which isresponsive to reflected voltage V1 757 across the primary winding 761 atthe input of energy transfer element 745.

The example embodiment illustrated in FIG. 7 therefore combines thecontrol strategies of power supplies 201 and 501. FIG. 8 illustratesseveral possible resulting output characteristics exhibited by variousembodiments of power supplies in accordance with the teachings of thepresent invention. For instance, at output power levels below themaximum power capability of the power supply 701, defined bycharacteristic curve 810, the power supply regulator 721 will regulatethe delivered output voltage to be substantially constant based on thecombined feedback current to terminal 725 through resistor 739 and optocoupler output transistor 741. When the maximum output power capabilityof the power supply 701 is reached, the current through opto-couplerinput LED 756 will reduce to substantially zero and the opto-coupleroutput transistor 741 will substantially turn off reducing the currentconducted in this path to substantially zero. The current feedback toterminal 725 of power supply regulator 721 is then providedsubstantially entirely through resistor 739. In one embodiment,characteristic curve 805 shows that one embodiment of a power supply 701in accordance with the teachings of the present invention has an outputcurrent that increases as output voltage decreases.

In yet another embodiment, characteristic curve 807 shows that anotherembodiment of a power supply 701 in accordance with the teachings of thepresent invention has an output current that decreases as output voltagedecreases.

In yet another embodiment, characteristic curve 808 shows that anotherembodiment of a power supply 701 in accordance with the teachings of thepresent invention has an output current that remains substantiallyconstant over the drop in output voltage.

In one embodiment, the design choices of resistors 739 and 740 determinethe value of the output current as the output voltage drops at outputpowers greater than the maximum power capability of the power supply701. In one embodiment the design choice of input LED 756 determines theoutput voltage value at output powers less than the maximum powercapability of the power supply 701. Characteristic curve 804 shows theoutput characteristic of one embodiment where the output current at themaximum output power is greater than the output current determined bythe design choice of resistors 739 and 740. The output characteristiccurve 804 shows that in this embodiment the output current will exceedthat determined by the design choice of resistors 739 and 740 until themaximum output power is reached whereupon the output current will reduceto the value determined by the choice of resistor 739 and 740.

In the embodiments illustrated, flyback converter power supplies havebeen provided for explanations of the present invention. It isappreciated that other power supply configurations such as for examplenon-isolated buck converter power supplies using for example inductorsfor energy transfer elements may also be utilized in accordance with theteachings of the present invention. Since the inductor used in thenon-isolated buck converter has only one winding which is coupled toboth input and output, the equivalent turns ratio is equal to 1 and thereflected voltage is the same as the output voltage.

In the foregoing detailed description, the present invention has beendescribed with reference to specific exemplary embodiments thereof. Itwill, however, be evident that various modifications and changes may bemade thereto without departing from the broader spirit and scope of thepresent invention. The present specification and figures are accordinglyto be regarded as illustrative rather than restrictive.

What is claimed is:
 1. A power supply, comprising: an energy transferelement having an energy transfer element input and an energy transferelement output; a power switch coupled between the energy transferelement input and a positive input supply rail; and control circuitrycoupled to the energy transfer element input and to the power switch,the control circuitry to switch the power switch in response to acurrent derived from the energy transfer element input.
 2. The powersupply of claim 1 further comprising an optocoupler including: an inputlight emitting diode (LED) coupled to the energy transfer element outputand an output of the power supply; and an output transistor coupled tothe control circuitry.
 3. The power supply of claim 2 wherein a currentthrough the output transistor is coupled to be received by the controlcircuitry and is responsive to the current derived from the energytransfer element input.
 4. The power supply of claim 1 wherein theenergy transfer element comprises a transformer having a primary windingcoupled to the energy transfer element input and a secondary windingcoupled to the energy transfer element output.
 5. The power supply ofclaim 4 wherein the voltage at the energy transfer element inputcomprises a reflected voltage across the primary winding.
 6. The powersupply of claim 1 wherein the power switch comprises a metal oxidesemiconductor field effect transistor (MOSFET).
 7. The power supply ofclaim 6 wherein the MOSFET is an n-channel MOSFET having a sourceterminal coupled to the energy transfer element input, a drain terminalcoupled to the positive input supply rail and a gate terminal coupled tothe control circuitry.
 8. The power supply of claim 4 further comprisinga first capacitor coupled in parallel with the primary winding during anoff cycle of the power switch, the first capacitor decoupled from theprimary winding during an on cycle of the power switch, the controlcircuitry coupled to the first capacitor, wherein the current derivedfrom the energy transfer element input is responsive to a voltage acrossthe energy transfer element output.
 9. The power supply of claim 1wherein the control circuitry comprises a pulse width modulator coupledto the power switch, the pulse width modulator to pulse width modulatethe power switch in response to the current derived from the energytransfer element input.
 10. The power supply of claim 1 wherein thecontrol circuitry comprises a on/off control circuitry coupled to thepower switch, the on/off control circuitry to adjust cycles of a controlsignal coupled to be received by the power switch in response to thecurrent derived from the energy transfer element input.
 11. The powersupply of claim 1 wherein the control circuitry disables switching ofthe power switch in response to the current derived from the energytransfer element input exceeding a threshold amount.
 12. The powersupply of claim 1 wherein the control circuitry and the power switch areincluded on a single monolithic chip having as few as three electricalterminals.